Motive power is transmitted in a variety of ways including by drive systems having electric motors or internal combustion engines coupled to gears, shafts and the like. One type of motive power system involves a driving pulley coupled to a prime mover such as an engine, e.g., an internal combustion engine. The driving pulley is coupled to a driven pulley by one or more continuous or endless belts, e.g., a belt of the type often referred to as a "V-belt" because of its cross-sectional shape.
In drive systems of this type, belt tension is an important application parameter. The belt must "grip" the pulley (typically by friction but, in some drives, by using pulleys and belts which have engaging gear-like teeth) to transmit needed power to the driven mechanism. In other words, the drive designer anticipates nonslipping engagement of belt and pulley so that the drive can do its job of transmitting horsepower.
Motor vehicles have motive power systems of the belt-and-pulley type in that the vehicle engine has a driving pulley coupled by a belt to one or more driven pulleys. The driven pulleys power such components as a cooling fan, alternator, power steering pump and, perhaps, other accessories. Typically, one of such components (often the alternator) is mounted on a swinging bracket so that the belt can be tightened to the desired tension.
Yet another example of a type of machine having a motive power system of the belt-and-pulley type is an agricultural grain combine. And one way in which such a combine employs a belt-and-pulley drive is with respect to the unloading tube which transfers harvested grain from the combine to a haulage vehicle. Movement of grain along the tube is by a rotary auger, the shaft of which is coupled to a driven pulley. A driving pulley, powered by the combine engine, is coupled to the driven pulley by an endless belt.
Belt-and-pulley drives are of two general types. In one type, the belt and pulleys are continuously engaged and the belt continuously in tension. The belt-and-pulley drives on an automobile are arranged this way since it is desired to operate all accessories whenever the engine is running. A system of this general type is shown in U.S. Pat. No. 5,176,580 (Stamm et al.).
In another type having an idler pulley (which is solely a belt tension-adjusting device), the belt is kept in tension by the idler pulley but can be "relaxed" when such pulley is moved out of the drive position. When the belt is relaxed, there is insufficient frictional engagement of the belt and the pulleys and the driven pulley stops rotating. A system arranged in this way is shown in U.S. Pat. No. 4,400,930 (Huhman et al.).
Almost irrespective of the particular application for which the belt-and-pulley drive system is used, such systems tend to be characterized by certain disadvantages. One disadvantage is that after exposure to tension and, probably, heat for an extended time, the drive belt tends to stretch slightly. In drives where a particular belt tension is required to transmit the required horsepower, a significant reduction in such tension will cause the belt to slip (to a lesser or greater degree), perhaps even enough to stop the drive.
Another disadvantage is that even with new drive belts nominally meeting a particular length specification, the overall length of the belt tends to vary modestly from belt to belt and/or from manufacturer to manufacturer. In either situation (involving stretched belts or new belts of slightly differing length), some means must be provided to maintain or regain desired belt tension.
One way to do so is by using tensioning springs, either per se as shown in French Patent ("Brevet D'Invention") No. 995.495 or within a housing as shown in U.S. Pat. No. 5,16,284 (Cho). However, a disadvantage of tensioning springs is inherent in the characteristics of a spring.
That is, a spring exerts a specified amount of force for each dimensional unit that the spring is compressed (or stretched) from its length of repose. For example, if a compression spring 5 inches long in repose and having a rating of 20 pounds per inch is compressed to a length of 3 inches (i.e., shortened by 2 inches), the spring exerts a force of 40 pounds (20 lbs./inch times 2 inches). If, because of belt stretch, the spring is permitted to extend to, say, 4 inches in length, it will then exert a force of only 20 pounds. Such inherent spring characteristics make it difficult to maintain substantially constant belt tension.
Another way to maintain desired belt tension is by using some sort of powered actuator. Offenlegungsschrift 27 07 247 (German patent document) shows an idler pulley mounted to a lever moved by an electrically powered linear actuator. U.S. Pat. Nos. 5,025,614 (Orsborn et al.) and 4,283,181 (Sproul) show belt tensioning arrangements involving hydraulic actuators.
The Sproul patent states that its constant-pressure actuator provides a constant tensioning force on the belt. Disregarding whether this assertion is correct (and given the depicted lever arrangement, there is certainly room for doubt), it overlooks the fact that constant tensioning force will not result in constant belt tension in situations involving belts having significantly disparate lengths. The reason underlying such fact will be apparent after appreciating the invention and the detailed description thereof.
An improved belt tensioning mechanism overcoming some of the problems and shortcomings of prior art arrangements and, particularly, providing substantially constant belt tension even with a belt having a significant variance in length would be an important advance in the art.